VCO with switchable varactor for low KVCO variation

ABSTRACT

A method and system for VCO with switchable varactor for low KVCO variation is provided. Aspects of a method for controlling a signal may comprise controlling an oscillating frequency of a time varying signal in a circuit, and controlling a change in the oscillating frequency based on switching of a plurality of varactors. The switching of the varactors may control a change in the oscillating frequency based on a control voltage. The method may further comprise minimizing a change in the rate of change in the oscillating frequency based on the switching of the varactors. Aspects of a system for controlling a signal may comprise circuitry that controls an oscillating frequency of a time varying signal in a circuit, and circuitry that controls a change in the oscillating frequency based on switching of a plurality of varactors. An executable computer program may cause a machine to execute steps as described above.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

Not Applicable.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to analog circuit design.More specifically, certain embodiments of the invention relate to amethod and system for a voltage controlled oscillator (VCO) withswitchable varactor for low KVCO variation.

BACKGROUND OF THE INVENTION

A circuit that generates a signal for which an oscillating frequency ofthe signal is proportional to an applied voltage may be known as avoltage controlled oscillator (VCO). A device for which the capacitancevalue varies based on an applied voltage may be known as a variablereactance, or varactor. The oscillating frequency of a VCO may becontrolled by utilizing a varactor. The value of KVCO may control theamount by which the oscillating frequency of a time varying signalgenerated by a VCO may change based on a change in the voltage level ofa control signal. In operation, the value of KVCO in a VCO may varywidely due to a plurality of factors. A change in KVCO may change therate at which the oscillating frequency of the VCO may change due tochanges in the control voltage. One or more of these factors may alsoresult in changes in the oscillating frequency of the time varyingsignal generated by the VCO independent from changes in the controlvoltage. In addition, an amplitude of the time varying signal may changedue to one or more of these factors. Some conventional VCO designs maynot be able to adapt the VCO circuitry to compensate for this pluralityof factors such to stabilize values of KVCO, the oscillating frequencyof the VCO for a given control voltage level, and the amplitude of thetime varying signal generated by the VCO.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method is provided for a VCO with switchable varactorsfor low KVCO variation, substantially as shown in and/or described inconnection with at least one of the figures, as set forth morecompletely in the claims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary phase locked loop circuit, inaccordance with an embodiment of the invention.

FIG. 2 is a chart illustrating exemplary oscillating frequency versuscontrol voltage, in accordance with an embodiment of the invention.

FIG. 3 is a block diagram of an exemplary system for VCO with switchablevaractor for low KVCO variation, in accordance with an embodiment of theinvention.

FIG. 4 is a diagram of an exemplary bank of varactors, which may beutilized in connection with an embodiment of the invention.

FIG. 5 a is a diagram of an exemplary switch implemented as a pluralityof single pole switches, which may be utilized in connection with anembodiment of the invention.

FIG. 5 b is a diagram of an exemplary switch implemented as a multi-poleswitch, which may be utilized in connection with an embodiment of theinvention.

FIG. 6 is an exemplary current source circuit design, which may beutilized in connection with an embodiment of the invention.

FIG. 7 is an exemplary positive feedback circuit design, which may beutilized in connection with an embodiment of the invention.

FIG. 8 is an exemplary circuit design for a VCO with switchable varactorfor low KVCO variation, in accordance with an embodiment of theinvention.

FIG. 9 is a flow chart illustrating an exemplary method for switchingvaractors in a VCI with switchable varactor for low KVCO variation, inaccordance with an embodiment of the invention.

FIG. 10 is a flow chart illustrating an exemplary method for unswitchingvaractors in a VCI with switchable varactor for low KVCO variation, inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention may be found in a method and systemfor VCO with switchable varactor for low KVCO variation. FIG. 1 is ablock diagram of an exemplary phase locked loop (PLL) circuit, inaccordance with an embodiment of the invention. Referring to FIG. 1there is shown a phase detector block 102, a charge pump block 104, aresistor 106, a plurality of capacitors 108 and 110, a voltagecontrolled oscillator (VCO) 112, a VCO control block 114, and a divideby N (+N) block 116. Also shown in FIG. 1 are a reference signal (Ref),a control voltage signal (V_(cntl)), a varactor control code, and timevarying signals output positive (O_(P)), and output negative (O_(N)).The resistor 106, and plurality of capacitors 108 and 110, may becomponents in a loop filter. The loop filter may be characterized by animpedance that varies as a function of frequency, Z_(loop)(s), where thevariable s may represent the frequency of a signal applied to the loopfilter.

In operation, a PLL may receive a reference signal, Ref, that is aninput to a phase detector 102. The phase detector 102 may compare thereference signal ref to the time varying signals that may be output fromthe VCO 112, O_(P) and O_(N). The phase detector 102 may output a signalbased on the phase difference between the reference signal, Ref, and thetime varying signals, O_(P) and O_(N). Prior to comparison at the phasedetector block 102, the time varying signals, O_(P) and O_(N), may beprocessed by the divide by N block 116. The output from the phasedetector may be received by the charge pump block 104, which maygenerate a current, i(s). The average direct current (DC) component ofthe current i(s) may be proportional to the phase difference between thereference signal, Ref, and the signal generated by the processing of thetime varying signals O_(P) and O_(N) by the divide by N block 116. Thecurrent generated by the charge pump block 104, i(s), may be applied tothe loop filter to produce a control voltage V_(cntl):V _(cntl) =Z _(loop)(s)i(s)  equation [1]

The control voltage signal, V_(cntl), may be input to the VCO 112. TheVCO 112 may generate the time varying signals, O_(P) and O_(N), based onthe control voltage signal, V_(cntl), and the varactor control code. Thevaractor control code may be generated by the VCO control block 114. TheVCO control block 114 may utilize machine readable storage having storedthereon, a computer program having at least one code section that isexecutable by a machine, which causes the machine to generate a varactorcontrol code. The varactor control code may comprise a plurality ofbinary bits.

FIG. 2 is a chart illustrating exemplary oscillating frequency versuscontrol voltage, in accordance with an embodiment of the invention. Withreference to FIG. 2, there is shown a plurality of graphs 202, 204, 206,208, 210, 212, 214, and 216. Each of the plurality of graphs 202, 204,206, 208, 210, 212, 214, and 216 may represent a series of measurementsof the oscillating frequency, F_(VCO), of a time varying signal, suchas, for example, O_(P) or O_(N), that is output from a VCO 112 based ona control voltage V_(cntl). The variable KVCO may be defined as thechange in F_(VCO) based on a change in V_(cntl):

$\begin{matrix}{{KVCO} = \frac{\mathbb{d}F_{VCO}}{\mathbb{d}V_{cntl}}} & {{equation}\mspace{14mu}\lbrack 2\rbrack}\end{matrix}$where KVCO may be represented in FIG. 2 based on the slopes of thegraphs 202, 204, 206, 208, 210, 212, 214, and 216.

In operation, the value of KVCO may vary from a nominal value,KVCO_(nominal), by 50% to 200%:0.5 KVCO_(nominal)≦KVCO≦2 KVCO_(nominal)  equation [3]A change in KVCO may represent a rate at which the oscillator frequencyof the VCO, F_(VCO), may change based on a change in the control voltageV_(cntl).

The value of KVCO may change based on a plurality of factors. Forexample, the value of KVCO may change based on the amplitude of the timevarying signals, O_(P) and O_(N), such that for large amplitudes of thetime varying signals, the value of KVCO may be smaller. In this regard,factors that cause a change in the amplitude of the time varyingsignals, O_(P) and O_(N), may induce a corresponding change in the valueof KVCO.

Various embodiments of the invention may control factors that cause achange in the amplitude of the time varying frequency signals, O_(P) andO_(N). Other aspects of the invention may control factors that may causea change in the oscillating frequency the signals O_(P) and O_(N), whichmay be generated by the voltage controlled oscillator (VCO).

FIG. 3 is a block diagram of an exemplary system for VCO with switchablevaractor for low KVCO variation, in accordance with an embodiment of theinvention. With reference to FIG. 3, there is shown a first bank ofvaractors 302, a second bank of varactors 304, a plurality of switches306 . . . 308, a plurality of inductors 310 and 312, a plurality of loadcapacitors 314 and 316, a current source circuit 318, and a positivefeedback circuit 320. Also shown in FIG. 3 is a bias current inputsignal, I_(bias), the time varying signals, O_(P) and O_(N), a controlvoltage V_(cntl), a supply voltage V_(dd), and a ground referencevoltage gnd. The ground reference voltage may also be referred to as aground reference, or ground.

The first bank of varactors 302 and the second bank of varactors 304 mayeach comprise a plurality of varactors. Each varactor in the first bankof varactors 302 may be coupled to a corresponding varactor in thesecond bank of varactors 304, such that, for example, the first varactorin the first bank of varactors 302 may be coupled to the first varactorin the second bank of varactors 304, and so forth. Each coupled pair ofvaractors in the first bank of varactors 302 and the second bank ofvaractors 304 may be coupled to a switch from the plurality of switches306 . . . 308. Each switch from the plurality of switches 306 . . . 308may be coupled to a control voltage V_(cntl), a supply voltage V_(dd),and a ground reference voltage gnd.

Each varactor in the first bank of varactors 302 may be coupled to aninductor 310, to a load capacitor 314, and to a terminal of a positivefeedback circuit 320. Each varactor in the second bank of varactors 304may be coupled to an inductor 312, to a load capacitor 316, and to adifferent terminal of a positive feedback circuit 320 from the terminalon the positive feedback circuit 320 to which the first bank ofvaractors may be coupled. The load capacitors 314 and 316 may be coupledto the ground reference voltage gnd. Each of the inductors 310 and 312may be coupled to a current source circuit 318. The current sourcecircuit 318 may be coupled to a bias current input signal, I_(bias), andto the supply voltage V_(dd).

FIG. 4 is a diagram of an exemplary bank of varactors, which may beutilized in connection with an embodiment of the invention. Withreference to FIG. 4, there is shown a bank of varactors 402. The bank ofvaractors 402 may comprise a plurality of varactors 404 . . . 406.Various embodiments of the invention may implement a varactor in theplurality of varactors 404 . . . 406 utilizing a variety of devices. Avaractor in the plurality of varactors 402 may, for example, beimplemented utilizing a diode. Alternatively, a varactor in theplurality of varactors 402 may be implemented utilizing a metal oxidesemiconductor field effect device (MOSFET).

FIG. 5 a is a diagram of an exemplary switch implemented as a pluralityof single pole switches, which may be utilized in connection with anembodiment of the invention. With reference to FIG. 5 a, there is showna switch 502. The switch 502 may comprise a plurality of single poleswitches 504, 506, 508, and 510. Individually, each of the single poleswitches 504, 506, 508, and 510 may couple a single input to a singleoutput. A single pole switch 504 may be coupled to each of the singlepole switches 506, 508, and 510. When the plurality of single poleswitches 504, 506, 508, and 510 are coupled as shown in FIG. 5 a, theswitch 502 may couple a single input to one of a plurality of outputs,or the switch 502 may couple a single output to one of a plurality ofinputs.

FIG. 5 b is a diagram of an exemplary switch implemented as a multi-poleswitch, which may be utilized in connection with an embodiment of theinvention. With reference to FIG. 5 b, there is shown a switch 512. Theswitch 512 may comprise a multi-pole pole switch 514. The multi-poleswitch 514 may couple a single input to one of a plurality of outputs,or the multi-pole switch 514 may couple a single output to one of aplurality of inputs. Various embodiments of the invention may utilize aswitch that comprises aspects of 502 or 512.

FIG. 6 is an exemplary current source circuit design, which may beutilized in connection with an embodiment of the invention. Withreference to FIG. 6, there is shown a current source circuit 602. Thecurrent source circuit 602 may comprise p-channel MOSFETs 604, and 606.Also shown in FIG. 6, is a bias current input signal, I_(bias). A MOSFETmay also be referred to as an FET, or transistor.

In operation, a bias current input signal, I_(bias), may be supplied tothe drain terminal, or drain, of the transistor 604. Associated with thebias current input signal, I_(bias), is a bias voltage, V_(bias), whichmay be applied to the gate terminal, or gate, of the transistor 604. Thedifference between the voltage applied between the gate of a transistor,and the voltage applied to the source terminal, of the source, of thesame transistor, may be referred to as the gate to source voltage,v_(gs). The drain current of a transistor, i_(d), such as, for exampletransistor 604, may be proportional to the gate to source voltage,v_(gs).

The current source circuit 602 may be referred to as a current mirror inthat the value of the gate to source voltage v_(gs) that is applied totransistor 604 may also equal the value of v_(gs) that is applied totransistor 606. Therefore, a percentage change in the current i_(d) fortransistor 604 may produce a comparable percentage change in the currenti_(d) for transistor 606. The actual value of the change in i_(d) fortransistor 604 may not equal the actual value of the change i_(d) in fortransistor 606. One reason may be that the value of the transconductanceg_(m) for transistor 604 may not equal the value of the transconductanceg_(m) for transistor 606.

FIG. 7 is an exemplary positive feedback circuit design, which may beutilized in connection with an embodiment of the invention. Withreference to FIG. 7, there is shown a positive feedback circuit 712. Thepositive feedback circuit 712 may comprise n-channel transistors 714,and 716.

In operation, a voltage at the drain of transistor 714 may be applied tothe gate of transistor 716. A voltage at the drain of transistor 716 maybe applied to the gate of transistor 714. The voltage at the source oftransistor 714 may be equal to the voltage at the source of transistor716. The impedance coupled to the drain of transistor 714 may bereferred to as Z₇₁₄(s), where the impedance Z₇₁₄(s) may vary based onthe oscillating frequency, s, of the signal applied at the drain oftransistor 714. The impedance coupled to the drain of transistor 716 maybe referred to as the impedance Z₇₁₆(s), where Z₇₁₆(s) may vary based onthe oscillating frequency, s, of the signal applied at the drain oftransistor 716.

The positive feedback in the positive feedback circuit 712 may result ina decrease of the voltage at the drain of a transistor, such as, forexample, transistor 714, leading to a further decrease in the samevoltage based on the feedback in the positive feedback circuit 712.Conversely, an increase in the voltage at the drain of a transistor maylead to a further increase in the same voltage based on the feedbackmechanism in the positive feedback circuit 712.

FIG. 8 is an exemplary circuit design for a VCO with switchable varactorfor low KVCO variation, in accordance with an embodiment of theinvention. With reference to FIG. 8, there are shown p-channeltransistors 802 and 804, inductors 806 and 808, n-channel transistors810 and 812, load capacitors 814 and 816, a plurality of varactors 818 .. . 820, and 822 . . . 824, and a plurality of switches 826 . . . 828.The plurality of varactors 818 . . . 820 may comprise a first bank ofvaractors, and the plurality of varactors 822 . . . 824 may comprise asecond bank of varactors. The transistors 802 and 804 may be componentsin a current source circuit 602. The transistors 810 and 812 may becomponents in a positive feedback circuit 712. Also shown in FIG. 8 is abias current input signal, I_(bias), the time varying signals, O_(P) andO_(N), a control voltage V_(cntl), a supply voltage V_(dd), and a groundreference voltage gnd.

A varactor in the plurality of varactors 818 . . . 820, and 822 . . .824 may be implemented as an n-channel MOSFET, or NMOS transistor, forwhich the drain is coupled to the source. The capacitance of a varactor,C_(var), in the plurality 818 . . . 820 may be based on the differencein the voltage level of the time varying signal, O_(P), applied to thegate of a varactor in the plurality 818 . . . 820, and the voltage levelcoupled to the drain and source of the same varactor in the plurality818 . . . 820 via a corresponding switch in the plurality of switches826 . . . 828. The capacitance of a varactor in the plurality 822 . . .824 may be based on the difference in the voltage level of the timevarying signal, O_(N), applied to the gate of a varactor in theplurality 822 . . . 824, and the voltage level coupled to the drain andsource of the same varactor in the plurality 822 . . . 824 via acorresponding switch in the plurality of switches 826 . . . 828.

When the voltage level of the time varying signal, O_(P), coupled to thegate of a varactor, implemented as an NMOS transistor, in the pluralityof varactors 818 . . . 820 is greater than the sum of the gate to sourcethreshold voltage level, and the voltage level coupled to the source anddrain of the same varactor, the capacitance of that varactor may reach amaximum capacitive value C_(max). When the voltage level of the timevarying signal, O_(P), coupled to the gate of a varactor, implemented asan NMOS transistor, in the plurality of varactors 818 . . . 820 is notgreater than the sum of the gate to source threshold voltage level, andthe voltage level coupled to the source and drain of the same varactor,the capacitance of that varactor may reach a minimum capacitive valueC_(min). In an exemplary normal NMOS transistor, the gate to sourcethreshold voltage level may approximately equal 600 millivolts. In anexemplary accumulation type NMOS transistor, the gate to sourcethreshold voltage level may approximately equal 0 volts.

The average value of the capacitance of a varactor implemented as anNMOS transistor, Avg(C_(var)), may increase as the proportion of timeincreases during which the time varying signal O_(P) is greater than thesum of the gate to source threshold voltage level, and the voltage levelcoupled to the source and drain of the same varactor. Therefore, if thevoltage level of V_(dd) is greater than the voltage levels of thecontrol voltage V_(cntl) and gnd, and if the voltage level of V_(cntl)is greater than the voltage level of the ground, gnd:Avg(C _(var)(gnd)>Avg(C _(var)(V _(cntl)))>Avg(C _(avg)(V_(dd)))  equation [4]where Avg(C_(var)(V_(dd))) may represent the average value of thecapacitance of a varactor when the drain and source are coupled to thesupply voltage V_(dd), Avg(C_(var)(V_(cntl))) may represent the averagevalue of the capacitance of a varactor when the drain and source arecoupled to the control voltage V_(cntl), and Avg(C_(var)(gnd)) mayrepresent the average value of the capacitance of a varactor when thedrain and source are coupled to the ground, gnd. If the voltage level ofV_(dd) is greater than the voltage level of the amplitude of the timevarying signal O_(P), then the Avg(C_(var)(V_(dd))) may be equal to theminimum capacitance C_(min). If the voltage level of the amplitude ofthe time varying signal O_(P) is greater than gnd, thenAvg(C_(var)(gnd)) may be equal to the maximum capacitance C_(max).

If the voltage level of the amplitude of the time varying signal O_(P)is small, then the Avg(C_(var)(V_(cntl))) may be more sensitive to smallchanges in V_(cntl) because small changes in the control voltageV_(cntl) may result in large changes in the portion of time during whichO_(P) is greater than the sum of the gate to source threshold voltagelevel, and the voltage level of the control voltage V_(cntl). If thevoltage level of the amplitude of the time varying signal O_(P) islarge, then the Avg(C_(var)(V_(cntl))) may be less sensitive to smallchanges in the control voltage V_(cntl) because small changes inV_(cntl) may not result in large changes in the portion of time duringwhich O_(P) is greater than the sum of the gate to source thresholdvoltage level, and the voltage level of the control voltage V_(cntl).

The value of the total capacitance, C_(tot,P), for the half of thevoltage controlled oscillator (VCO) comprising the second bank ofvaractors 818 . . . 820 may be represented by:C _(tot,P) =C _(load) +iC _(min) +jC _(max) +kAvg(C _(var)(V_(cntl)))  equation [5]where i may represent the number of varactors among the plurality ofvaractors 818 . . . 820 that are coupled to the supply voltage V_(dd), jmay represent the number of varactors among the plurality of varactors818 . . . 820 that are coupled to the ground, gnd, and k may representthe number of varactors among the plurality of varactors 818 . . . 820that are coupled to the control voltage V_(cntl). C_(load) may representthe capacitance of the load capacitor 814. Among the components ofC_(tot,P) in equation [5], the value of C_(var)(V_(cntl)) may vary basedon the control voltage V_(cntl).

When the voltage level of the time varying signal, O_(N), coupled to thegate of a varactor implemented as an NMOS transistor in the plurality ofvaractors 822 . . . 824 is greater than the sum of the gate to sourcethreshold voltage level, and the voltage level coupled to the source anddrain of the same varactor, the capacitance of that varactor may reach amaximum capacitive value C_(max). When the voltage level of the timevarying signal, O_(N), coupled to the gate of a varactor in theplurality of varactors 822 . . . 824 is not greater than the sum of thegate to source threshold voltage level, and the voltage level coupled tothe source and drain of the same varactor, the capacitance of thatvaractor may reach a minimum capacitive value C_(min).

The average value of the capacitance of a varactor implemented as anNMOS transistor, Avg(C_(var)), may increase as the proportion of timeincreases during which the time varying signal O_(N) is greater than thesum of the gate to source threshold voltage level, and the voltage levelcoupled to the source and drain. Therefore, if the voltage level ofV_(dd) is greater than the voltage levels of V_(cntl) and gnd, and ifthe control voltage V_(cntl) is greater than the voltage level of theground, gnd, the relationship between Avg(C_(var)(V_(dd))),Avg(C_(var)(V_(cntl))), and Avg(C_(var)(gnd)) may be as expressed inequation [4]. If the voltage level of the supply voltage V_(dd) isgreater than the voltage level of the amplitude of the time varyingsignal O_(N), then the Avg(C_(var)(V_(dd))) may be equal to the minimumcapacitance C_(min). If the voltage level of the amplitude of the timevarying signal O_(N) is greater than ground, gnd, then Avg(C_(var)(gnd))may be equal to the maximum capacitance C_(max).

If the voltage level of the amplitude of the time varying signal O_(N)is small, then the Avg(C_(var)(V_(cntl))) may be more sensitive to smallchanges in the control voltage V_(cntl) because small changes inV_(cntl) may result in large changes in the portion of time during whichO_(N) is greater than the sum of the gate to source threshold voltagelevel, and the voltage level of the control voltage V_(cntl). If thevoltage level of the amplitude of the time varying signal O_(N) islarge, then the Avg(C_(var)(V_(cntl))) may be less sensitive to smallchanges in the control voltage V_(cntl) because small changes inV_(cntl) may not result in large changes in the portion of time duringwhich O_(N) is greater than the sum of the gate to source thresholdvoltage level, and the voltage level of the control voltage V_(cntl).

The value of the total capacitance, C_(tot,N), for the half of thevoltage controlled oscillator (VCO) comprising the second bank ofvaractors 822 . . . 824 may be represented by:C _(tot,N) =C _(load) +lC _(min) +mC _(max) +nAvg(C _(var)(V_(cntl)))  equation [6]where l may represent the number of varactors among the plurality ofvaractors 822 . . . 824 that are coupled to the supply voltage V_(dd), mmay represent the number of varactors among the plurality of varactors822 . . . 824 that are coupled to the ground, gnd, and n may representthe number of varactors among the plurality of varactors 822 . . . 824that are coupled to the control voltage V_(cntl). C_(load) may representthe capacitance of the load capacitor 816. Among the components ofC_(tot,N) in equation [6], the value of C_(var)(V_(cntl)) may vary basedon the control voltage V_(cntl).

In addition to voltage, value of the capacitance of the varactor,C_(var), may depend upon other factors. The amount of capacitance of avaractor when implemented utilizing a MOSFET may be based on thephysical geometry of the transistor. Manufacturing, or operatingtemperature variations among varactors, and C_(ox) variations, mayresult in variations in the value of the capacitance among the pluralityof varactors 818 . . . 820, and 822 . . . 824. Differences in thresholdvoltages among the varactors may also result in variations in the valueof the capacitance among the plurality of varactors 818 . . . 820, and822 . . . 824.

The inductors 806 and 808, the load capacitors 814 and 816, and thefirst and second banks of varactors 818 . . . 820 and 822 . . . 824, maycomprise a tank which may implement a bandpass filter. The currentsource circuit, comprising transistors 802 and 804, may supply energy tothe tank that generates time varying signals O_(P) and O_(N) with anoscillating frequency F_(osc) that may be expressed as:

$\begin{matrix}{F_{osc} = \frac{2\;\pi}{\sqrt{L_{tot}C_{tot}}}} & {{equation}\mspace{14mu}\lbrack 7\rbrack}\end{matrix}$where L_(tot) may represent the total inductance value of the inductors806 and 808, and C_(tot) may represent the sum of the capacitance valuesof the load capacitor 814 and 816, and of each varactor in the first andsecond banks of varactors 818 . . . 820 and 822 . . . 824, as expressedin equations [5] and [6]. The total inductance value L_(tot) may beexpressed as:L _(tot) =L ₈₀₆ +L ₈₀₈  equation [8]where L₈₀₆ may represent the inductance value of the inductor 806, andL₈₀₈ may represent the inductance value of the inductor 808. The totalcapacitance value C_(tot) may be expressed as:

$\begin{matrix}{C_{tot} = \frac{C_{{tot},N}C_{{tot},P}}{C_{{tot},N} + C_{{tot},P}}} & {{equation}\mspace{14mu}\lbrack 9\rbrack}\end{matrix}$where C_(tot,P) may represent the capacitance of the first bank ofvaractors as expressed in equation [5], and C_(tot,N) may represent thecapacitance of the second bank of varactors as expressed in equation[6]. For the capacitance value of C_(tot,P) approximately equal to thecapacitance value of C_(tot,N), the capacitance value of C_(tot) may beapproximately equal to ½C_(tot,P), or ½C_(tot,N).

The oscillating frequency, F_(osc), may represent the center frequencyin the pass band of the bandpass filter implemented by the tank. Thevalue of the center frequency divided by the bandwidth of the bandpassfilter may represent the Q factor, or Q, of the tank. The Q factor forthe tank may be expressed as:

$\begin{matrix}{Q = \frac{2\;\pi\; F_{osc}L}{R_{s}}} & {{equation}\mspace{14mu}\lbrack 10\rbrack}\end{matrix}$where R_(s) may represent the parasitic resistance associated with theinductors 806 and 808.

The parasitic resistance, R_(s), may dissipate the energy of the timevarying signals O_(P) and O_(N), with an oscillating frequency, F_(osc),that may be generated in the tank. The positive feedback circuit,comprising transistors 810 and 812, may supply energy that reinforcesthe frequency oscillation in the tank. As a result, the tank may be ableto sustain the time varying signals O_(P) and O_(N) with an oscillatingfrequency F_(osc).

The value of Q may influence the amplitude of the time varying signalsO_(P) and O_(N). A change in the bias current supplied by the currentsource circuit may increase the amount of energy to support frequencyoscillation in the tank. This may also change the amplitude of the timevarying signals O_(P) and O_(N). A change in the amplitude of the timevarying signals O_(P) and O_(N) may produce a change the value KVCO.Because the oscillating frequency, F_(osc), may be dependent on C_(tot),the factors which may result in variations in the capacitance of avaractor in the first and second banks of varactors 818 . . . 820, and822 . . . 824, may also change the oscillating frequency, F_(osc), ofthe time varying signals O_(P) and O_(N), which are output from thevoltage controlled oscillator (VCO).

A switch among the plurality of switches 826 . . . 828 may couple acorresponding varactor in the plurality of varactors 818 . . . 820, anda corresponding varactor in the plurality of varactors 822 . . . 824, toa supply voltage, V_(dd), a control voltage, V_(cntl), or a groundvoltage, gnd. The level of capacitance in varactors that are coupled toV_(dd) or gnd voltage levels in the first bank of varactors, and in thesecond bank of varactors may not vary based on changes in the controlvoltage V_(cntl). The level of capacitance in varactors that are coupledto the control voltage V_(cntl) in the first bank of varactors, and inthe second bank of varactors may vary based on changes in V_(cntl). Asthe number of varactors coupled to the control voltage V_(cntl) isincreased, a greater portion of the total capacitance in the tank,C_(tot), may vary based on V_(cntl). This, in turn, may result in theoscillating frequency for the tank, F_(osc), changing more rapidly for agiven change in the control voltage V_(cntl). According to equation [2],the change in an oscillating frequency based on a change in V_(cntl) maybe defined as the rate of change in the oscillating frequency, which maybe represented by KVCO. Thus, by utilizing the switches in the pluralityof switches 826 . . . 828, to couple varactors to the control voltageV_(cntl) in the first bank of varactors 818 . . . 820, and in the secondbank of varactors 822 . . . 824, the value of KVCO may be tuned.

Similarly, the value of total capacitance in the tank C_(tot), may betuned to compensate for other variations that may affect KVCO such as,for example, C_(ox) variation, threshold voltage variation, inductancevalue variation in the inductors 806 and 808, and operating temperaturevariation. Each switch in the plurality of switches 826 . . . 828 may becontrolled utilizing a plurality of binary bits. The binary value of theplurality of binary bits may be based on a varactor control code. Forexample, for variations that result in an increase in C_(tot), theswitches in the plurality of switches 826 . . . 828 may be controlledsuch that the number of varactors in the first and second banks ofvaractors, 818 . . . 820 and 822 . . . 824, which are coupled to theground voltage, gnd, may be decreased, and the number of varactors inthe first and second banks of varactors, 818 . . . 820 and 822 . . .824, which are coupled to the supply voltage, V_(dd), may be increased.With reference to equations [5] and [6], i and l may be decreased and jand m may be increased. As a result, the oscillating frequency, F_(osc)of the time varying signals O_(P) and O_(N) from the tank of the VCO maybe controlled.

For variations that result in a decrease in the total capacitance in thetank, C_(tot), the switches in the plurality of switches 826 . . . 828may be controlled such that the number of varactors, in the first andsecond banks of varactors, 818 . . . 820 and 822 . . . 824, which arecoupled to the ground voltage, gnd, may be increased, and the number ofvaractors, in the first and second banks of varactors 818 . . . 820 and822 . . . 824, which are coupled to V_(dd) may be decreased. Withreference to equations [5] and [6], i and l may be increased and j and mmay be decreased. As a result, the oscillating frequency, F_(osc) of thetime varying signals, O_(P) and O_(N), from the tank of the VCO may becontrolled.

FIG. 9 is a flow chart illustrating an exemplary method for switchingvaractors in a VCO with switchable varactor for low KVCO variation, inaccordance with an embodiment of the invention. With reference to FIG.9, in step 902, values for the oscillating frequency F_(osc), and KVCOmay be set. In step 904, the oscillating frequency F_(osc) and KVCO fora VCO 112 may be detected. In step 906, the detected F_(osc) and KVCOfrom step 904 may be evaluated to determine if they are within a targetrange determined in step 902. If F_(osc) and KVCO are within range, thecurrent varactor switch settings may be maintained.

In step 910, a varactor switching strategy may be determined if F_(osc)and KVCO are not within the target range in step 906. Step 912 maydetermine if any varactors are to be coupled to the supply voltage,V_(dd). Step 914 may couple the selected varactors to V_(dd) if step 912determined that varactors are to be coupled to V_(dd) that were notpreviously coupled to V_(dd). Step 916 may determine if any varactorsare to be coupled to the control voltage, V_(cntl). Step 918 may couplethe selected varactors to V_(cntl), if step 916 determined thatvaractors are to be coupled to V_(cntl) that were not previously coupledto V_(cntl). Step 920 may determine if any varactors are to be coupledto the ground voltage, gnd. Step 922 may couple the selected varactorsto gnd if step 920 determined that varactors are to be coupled to gndthat were not previously coupled to gnd.

FIG. 10 is a flow chart illustrating an exemplary method for unswitchingvaractors in a VCO with switchable varactor for low KVCO variation, inaccordance with an embodiment of the invention. With reference to FIG.10, in step 1002, values for the oscillating frequency F_(osc), and KVCOmay be set. In step 1004, the oscillating frequency F_(osc) and KVCO fora VCO 112 may be detected. In step 1006, the detected F_(osc) and KVCOfrom step 1004 may be evaluated to determine if they are within a targetrange determined in step 1002. If F_(osc) and KVCO are within range, thecurrent varactor switch settings may be maintained.

In step 1010, a varactor switching strategy may be determined if F_(osc)and KVCO are not within the target range in step 1006. Step 1012 maydetermine if any varactors are to be decoupled from the supply voltage,V_(dd). Step 1014 may decouple the selected varactors from V_(dd) ifstep 1012 determined that varactors are to be decoupled from V_(dd) thatwere previously coupled to V_(dd). Step 1016 may determine if anyvaractors are to be decoupled from the control voltage, V_(cntl). Step1018 may decouple the selected varactors from V_(cntl) if step 1016determined that varactors are to be decoupled from V_(cntl) that werepreviously coupled to V_(cntl). Step 1020 may determine if any varactorsare to be decoupled from the ground voltage, gnd. Step 1022 may decouplethe selected varactors from gnd if step 1020 determined that varactorsare to be decoupled from gnd that were previously coupled to gnd.

Various embodiments of the invention may provide a method and system fora voltage controlled oscillator (VCO) with switchable varactors for lowKVCO variation. In various embodiments of the invention, a totalcapacitance in a circuit may be modified to compensate for variations inVCO circuitry based on the switching of varactors. As a result, a changein KVCO may be minimized. Since KVCO may represent the change in theoscillating frequency of the VCO based on the change in a controlvoltage, minimizing the change in KVCO may minimize the variation of thephase locked loop bandwidth. Minimizing the change in KVCO may enablethe oscillating frequency to be generated more predictably based on thelevel of a control voltage than may be the case in some conventional VCOcircuit designs. Adapting the value of the capacitance in a VCO byswitching a plurality of varactors may also control the oscillatingfrequency of the time varying signal generated by the VCO, in additionto controlling a change in the oscillating frequency that may resultfrom variations in the VCO circuitry.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

1. A method for controlling a signal, the method comprising: configuringat least one varactor of a plurality of varactors by selecting a voltagelevel from a plurality of voltage references comprising a supplyvoltage, a control voltage, and a ground reference voltage; controllingan oscillating frequency of a time varying signal in a circuit based onsaid configuring; and controlling a rate of change in said oscillatingfrequency based on said configuring.
 2. The method according to claim 1,wherein said switching of said plurality of varactors controls a changein said oscillating frequency based on a control voltage.
 3. The methodaccording to claim 1, comprising minimizing a change in said rate ofchange in said oscillating frequency due to an amplitude of said timevarying signal based on said switching of said plurality of varactors.4. The method according to claim 1, comprising minimizing a change insaid oscillating frequency by adjusting a value for KVCO based on saidswitching of said plurality of varactors.
 5. The method according toclaim 1, comprising controlling said change of said time varying signalbased on a control voltage.
 6. The method according to claim 1,comprising controlling a capacitance in said circuit based on saidswitching of said plurality of varactors.
 7. A machine-readable storagehaving stored thereon, a computer program having at least one codesection for controlling a signal, the at least one code section beingexecutable by a machine for causing the machine to perform stepscomprising: configuring at least one varactor of a plurality ofvaractors by selecting a voltage level from a plurality of voltagereferences comprising a supply voltage, a control voltage, and a groundreference voltage; controlling an oscillating frequency of a timevarying signal in a circuit based on said configuring; and controlling arate of change in said oscillating frequency based on said configuring.8. The machine-readable storage according to claim 7, comprising codefor minimizing a change in said rate of change in said oscillatingfrequency due to an amplitude of said time varying signal based on saidswitching of said plurality of varactors.
 9. The machine-readablestorage according to claim 7, comprising code for minimizing a change insaid oscillating frequency by adjusting a value for KVCO based on saidswitching of said plurality of varactors.
 10. The machine-readablestorage according to claim 7, comprising code for controlling saidchange of said time varying signal based on a control voltage.
 11. Themachine-readable storage according to claim 7, comprising code forcontrolling a capacitance in said circuit based on said switching ofsaid plurality of varactors.
 12. A system for controlling a signal, thesystem comprising: circuitry that configures at least one varactor of aplurality of varactors by selecting a voltage level from a plurality ofvoltage references comprising a supply voltage, a control voltage, and aground reference voltage; said circuitry controls an oscillatingfrequency of a time varying signal in a circuit based on saidconfiguring; and said circuitry controls a rate of change in saidoscillating frequency based on said configuring.
 13. The systemaccording to claim 12, wherein said switching of said plurality ofvaractors controls a change in said oscillating frequency based on acontrol voltage.
 14. The system according to claim 12, wherein saidcircuitry minimizes a change in said rate of change in said oscillatingfrequency due to an amplitude of said time varying signal based on saidswitching of said plurality of varactors.
 15. The system according toclaim 12, wherein said circuitry minimizes a change in said oscillatingfrequency by adjusting a value for KVCO based on said switching of saidplurality of varactors.
 16. The system according to claim 12, whereinsaid circuitry controls said change of said time varying signal based ona control voltage.
 17. The system according to claim 12, wherein saidcircuitry controls a capacitance in said circuit based on said switchingof said plurality of varactors.
 18. The system according to claim 12,wherein said control voltage comprises a range of voltage levels. 19.The system according to claim 18, wherein said range of voltage levelscomprises 0 volts to 3 volts.
 20. A system for controlling a signal, thesystem comprising: a first bank of varactors comprising a firstplurality of varactors; a second bank of varactors comprising a secondplurality of varactors wherein each varactor in said second bank iscoupled to a corresponding one of said first plurality of varactors insaid first bank of varactors; a first of a plurality of switches throughwhich a corresponding first varactor in said first bank of varactors anda corresponding first varactor in said second bank of varactors arecoupled to one of a plurality of voltage references comprising a supplyvoltage, a control voltage, and a ground reference voltage; and asubsequent one of said plurality of switches through which acorresponding subsequent varactor in said first bank of varactors and acorresponding subsequent varactor in said second bank of varactors arecoupled to one of said plurality of voltage references.
 21. The systemaccording to claim 20, comprising coupling at least one of a pluralityof inductors to said first bank of varactors and a subsequent at leastone of said plurality of inductors to said second bank of varactors. 22.The system according to claim 21, comprising coupling a positivefeedback circuit to said at least one of said plurality of inductors,and to said subsequent at least one of said plurality of inductors. 23.The system according to claim 21, comprising coupling a current sourcecircuit to said plurality of inductors.